A customer had a question, forwarded to the operating system group via the customer liaison.
"My customer is developing an IDE and they want to implement the IntelliSense feature. We have been unable to find any documents explaining how to do this. Is there an API for this, or can you recommend a third-party SDK?"
IntelliSense isn't really an operating system feature. This is the sort of thing we call "business logic": The part of the program that is devoted to doing the things the program does, that only the program knows how to do. The operating system doesn't know how your business logic works; that's entirely within you control as the developer.
Windows doesn't really know anything about the syntax of C or C++ or Python or Java or whatever language your IDE is trying to support. Your IDE is going to have to parse the language and figure out what types the objects are and then offer appropriate suggestions based on what it has inferred. (This gets particularly tricky because you have to offer suggestions on code that probably isn't yet fully syntactically valid.)
"How should the customer get going if they accepted an order to develop an IntelliSense feature?"
Okay, wait, let me see if I understand this.
Your customer signed a contract promising to deliver an IntelliSense feature, but they don't have any idea where to start, and they're asking Microsoft to help them either by providing an API, or helping them find somebody who has a ready-made solution.
Apparently, this is what happens when people who grew up with StackOverflow get a job.
The customer needs to grab a blank piece of paper and start designing some software.